A first step in studying the molecular mechanisms involved in the renal secretion of organic cations has been accomplished, namely, the isolation of a protein that most likely is associated with this process. The direction that future research will take is: 1) to demonstrate that this protein is involved in transport. To accomplish this the following experimental approach will be used: a) binding studies with a series of organic cations and correlate the in vitro results with established in vivo data. b) Monitor binding of the ligand by following a change in some physical property of the protein, e.g. fluorescence emission, CD, ORD, c) Prepare an antibody against the purified protein. d) Chemical or enzymatic modification and the correlation of the modification of the protein with the loss of organic cation transport. 2) Chemical and physical characterization of the carrier protein. 3) Study the reabsorption of organic bases by microperfusion.